1. Field of the Invention
This invention relates to a process for the manufacture of tinplates. More particularly, this invention relates to a process to the manufacture of tinplates at a high speed.
Furthermore, this invention relates to a manufacturing equipment of tin-plated steel strip.
2. Description of the Prior Art
Tin plated steel sheets called tinplates have been well known and are widely used for the manufacture of tableware, containers, decorations, and the like. In recent years, they are extensively used for the manufacture of soldered wiring parts of electrical appliances.
Until now, a hot dipping process or an electroplating process has been used as a tin plating process.
The hot dipping process is suitable for the tin plating of a substrate which requires a large amount of coating, and the substrate is plated by immersion in a bath containing fused metallic tin.
The electroplating process is suitable for the tin plating of a substrate which requires a relatively small amount of coating thereon, and the substrate is electroplated as a cathode in an aqueous electroplating bath containing sodium stannate for Alkali Process, stannous sulfate and phenolsulfonic acid for Ferrostan Process, and stannous chloride, sodium chloride and sodium fluoride for Halogen Process.
The tinplate is manufactured in a large scale by the electroplating process because the amount of coating is a relatively small.
With the widespread use of automatic vending machines, the production volume of tinplates for use in the making of seamless DI cans for canned drinks is increasing rapidly in recent years. Accordingly, there is a need for developing a high-speed tinplate manufacturing process.
In the conventional hot dipping process, it is impossible to further increase the passing speed of the steel sheet in the bath containing fused metallic tin having a specific gravity of 7.28, and also impossible to perform the plating at high speed without impairment of the uniform coating of tin, because the amount of coatings is controlled by wringer roll governing.
At present, tinplates are manufactured by an electroplating process. In this case, the usable upper limit of current density is low, and limited to about 10 A/dm.sup.2 for Alkali Process, about 30 A/dm.sup.2 for Ferrostan Process, and about 50 A/dm.sup.2 for Halogen Process. Thus, when a high-speed manufacturing process is desired, it is necessary to install a number of plating baths additionally. That is expensive. Therefore, the maximum plating rate is limited to about 600 m/min or lower in the conventional plating facilities.
In order to achieve a high-speed manufacture of tinplates, it is necessary to develop a tin electroplating process at high current densities.
As the tin plating process which may operate the bath at a current density of 50 A/dm.sup.2 or higher, a fused-salt tin plating bath has been proposed.
Soviet Patent No. 109486 discloses a fused-salt tin plating tin plating bath containing SnCl.sub.2 -KCl, SnCl.sub.2 -KCl-ZnCl.sub.2, or SnCl.sub.2 -ZnCl.sub.2, which may operate at 200.degree. to 500.degree. C. and a current density of 50 to 100 A/dm.sup.2. However, the plating rate is only about twice as fast as it is in the conventional Halogen Bath and still insufficient to perform the high-speed plating.
Generally, tin-plated steel sheets are manufactured by discontinuous treatment of cold rolling steel sheets in the following individual apparatus and order named:
(a) continuous pretreating and annealing apparatus shown in FIG. 3-(a) PA1 (b) skin pass rolling apparatus shown in FIG. 3-(b) PA1 (c) tin-electroplating and aftertreating apparatus shown in FIG. 3-(c)
In recent years, there is an instance wherein the continuous annealing apparatus is integrated with the skin pass rolling apparatus into a new equipment for reducing the cost. In this case, there is substantially no difference in the proceeding speed of the steel sheet to be treated through the processes.
The cost will be further reduced, provided that the tin-electroplating apparatus can be further combined with the equipment. However, it is difficult to do so, because the proceeding speed of steel sheet in the plating process is too low in comparison with the speed of the sheet treated in the annealing or skin pass rolling process.
In the tin-electroplating process heretofore in use, the steel sheet is plated as a cathode in an aqueous tin-plating plating bath containing sodium stannate for Alkali Process, stannous sulfate and phenolsulfonic acid for Ferrostan Process, or stannous chloride, sodium chloride and sodium fluoride for Halogen Process [A Handbook of Iron and Steel, the 3rd. edition, vol. VI, 403].
In these processes, the usable current density has its upper limit of about 50 A/dm.sup.2 or lower, because of the low electrical conductivity of the aqueous plating bath and the burnt deposit of tin. Moreover, the proceeding speed of the sheet in the conventional plating bath has its upper limit of about 300 to 600 m/min. The speed is too low and cannot compare with the proceeding speed of about 600 to 900 m/min of the steel sheet in the continuous annealing or skin pass rolling process.
The tin-electroplating operation at a proceeding speed of about 600 to 900 m/min is impossible to realize because such a high-speed operation requires considerable number of plating baths, a large plant area and a larger construction cost unpreferably.
The object of the invention is to provide a process for the manufacture of tinplates including both reflow type and no-reflow type tinplates at high speed and high current densities.
Another object of the present invention is to provide a manufacturing equipment of tin-electroplated steel strip by which the manufacturing cost may be steeply reduced.